1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of gas such O.sub.2, NOx, or CO, and more particularly to an improved structure of a ceramic heater used in gas sensors and a manufacturing method thereof.
2. Background Art
FIGS. 1(a) and 1(b) show one example of conventional ceramic heaters which is built in an oxygen sensor for use in air-fuel ratio control of automotive internal combustion engines. The ceramic heater 9 serves to heat a sensor element up to an elevated temperature to minimize a variation in measured value.
The ceramic heater 9 consists of a ceramic square rod 10 made of a laminate of heater substrates and a covering substrate and metallic terminals 3 mounted on side surfaces 15 of the rod 10. The metallic terminals 3 connect electrically with leads of a heater-patterned layer in the rod 10 and joined to outer leads 4 through solders 5, respectively.
In manufacturing the ceramic heater 9, green sheets 101 and 102, as shown in FIG. 2(a), whose main component is alumina are first prepared. Next, a conductive paste is applied to the surface of each of the green sheets 101 to form a heater-patterned layer 2 consisting of pairs of a heater element 21 and a lead 22. The two green sheets 101 and the covering green sheet 102 are laid to overlap each other to form a three-layer laminate. The three-layer laminate is cut into several pieces as shown in FIG. 2(b). The metallic terminals 3 are formed on the side surfaces 15 of each piece which communicate electrically with the leads 22 to make an intermediate. Subsequently, the intermediate is baked, after which the outer leads 4 is, as shown in FIG. 2(c), welded to the metallic terminals 3 through the solder 5. Finally, welded portions of the outer leads 4 are, as indicated at numeral 6 in FIG. 1(b), plated with Ni to make the ceramic heater 9.
The above ceramic heater 9 and the manufacturing method thereof, however, have the following drawbacks.
The metallic terminals 3 are, as described above, mounted on the side surfaces 15 of the ceramic heater 9. It is, thus, only possible to attach the metallic terminals 3 to the square rod 10 after the three-layer laminate is cut as shown in FIG. 2(b). In other words, a large number of terminal attachment processes are required in mass-production of ceramic heaters.
In addition, the performance of the ceramic heater 9 is usually inspected after the outer leads 4 are mounted thereon. A large number of individual inspections are also required in the mass-production of ceramic heaters, thus resulting in an increase in manufacturing cost.
Another problem is also encountered in that the ceramic heater 9 is lower in durability than a round rod heater 91 as shown in FIG. 3(a). The results of heat cycle tests show that portions of the ceramic heater 9 welded to the outer leads 4 and the metallic terminals 3 tend to be cracked as compared with the round rod heater 91. This is because the angle .beta. which each of the metallic terminals 3 of the ceramic heater 9, as shown in FIG. 4, makes with the outer surface of the solder 5 is greater than the angle .alpha. which each of the metallic terminals 3 of the round rod heater 91, as shown in FIG. 3(b), makes with the outer surface of the solder 5. The difference between the angles .alpha. and .beta. depends upon the geometry of the heaters 9 and 91 and thus is difficult to eliminate. The use of solder which is soft enough to absorb internal stress ensures substantially the same durability of the portions of the rod 10 welded to the leads 4 as that of the round rod heater 91, however, square rod heaters exhibiting higher durability even in use of harder solder is sought.